Variable Length Subnet Masking (VLSM) is a way of further subnetting a subnet. Using Variable Length Subnet Masking (VLSM) we can allocate IPv4 addresses to the subnets by the exact need. Variable Length Subnet Masking (VLSM) allows us to use more than one subnet mask within the same network address space. If we recollect from the previous lessons, we can divide a network only into subnets with equal number of IPv4 addresses. Variable Length Subnet Masking (VLSM) allows to create subnets from a single network with unequal number of IPv4 addresses.

Example: We want to divide 192.168.10.0, which is a Class C network, into four networks, each with unequal number of IPv4 addresses requirements as shown below.

Now we have split the 192.168.10.0/24 network into four subnets using Variable Length Subnet Masking (VLSM), with unequal number of IPv4 addresses as shown below. Also note that when you divide a network using Variable Length Subnet Masking (VLSM), the subnet masks are also different.